In electrophotographic imaging processes, such as in electrophotographic copying machines an electrostatic latent-image charge pattern is formed on the photoconductive element which includes a photoconductive layer deposited on a conductive support and can be in the form of a belt, drum or plate. By treating the charge pattern with a dry developer containing charged toner particles, the latent image is developed. The toner pattern is then transferred to a receiver such as a sheet of paper to which it is fixed by fusion or other means.
In the most effective modern photocopiers, the active layers of the photoconductive element comprises organic charge generation or charge transport materials dispersed in a binder resin matrix. To permit long, continuous use of these photoconductive elements, the binder resin must be tough and strong. A problem, however, in transferring the developed image to a receiver is that the attraction of the toner to the surface layer of electrophotographic elements which employ the usual kinds of tough organic binder resins can cause incomplete transfer of toner. The resulting transferred image on the receiver has hollow characters and other defects. The problem is especially severe when the image is transferred by pressing a receiver element such as a paper sheet into contact with the tone surface of the photoconductive element.
Efforts to solve the image transfer problem have included providing abhesive or release coatings to the surface layers of photoconductive elements. A drawback of this attempt to solve the problem is that an insulating, non-photoconductive overcoat can interfere with the photoconductive properties of the element. If the coating is thick, it can reduce the electrophotographic speed or sensitivity. Even if thin, an insulating overcoat layer can shorten the life of a photoconductive film to such an extent that it cannot be regenerated for repeated use. This is believed to be caused by the trapping of residual charges between the insulating coating and the active surface layer. If the surface layer is merely coated with a soft release substance such as a metal stearate, the coating rapidly wears off and the transfer problem reappears. There is a need, therefore, for a binder composition for the surface layer of photoconductive elements which provides suitable surface properties for good image transfer without the necessity for release overcoats and yet which also has the physical strength required of binders in reusable photoconductive elements.
In addition to the need for a binder composition having good toner image transfer properties and good physical strength, there is also a need for such a composition that is soluble in volatile coating solvents and that is compatible with phthalocyanine photoconductive pigments. The latter are of particular importance in photoconductive elements having sensitivity to infra-red radiation and, hence, utility in recording the output of light emitting diodes and lasers. Pigments of this class do not disperse uniformly in many otherwise suitable binder resins. Accordingly, a binder resin matrix composition having the combination of physical strength, good image transfer capability and compatibility with photoconductive pigments has been needed.